PROVENANCE OF UPPER JURASSIC TO LOWER PALEOCENE STRATA NEAR THE BLACK HILLS UPLIFT, SOUTH DAKOTA

Provenance of Upper Jurassic to Lower Paleocene strata near the Black Hills Uplift, South Dakota

Our objective is to use detrital zircon U-Pb geochronology to determine the provenance of Upper Jurassic to Lower Paleocene strata deposited in the north-central United States around the present day Black Hills Uplift. We analyzed seven samples from the Upper Jurassic Unkpapa Sandstone (Morrison Formation equivalent), Lower Cretaceous Lakota Formation and Fall River Sandstone, Upper Cretaceous New Castle Sandstone and Fox Hills Sandstone, and Lower Paleocene Fort Union Formation. The Unkpapa Sandstone, Morrison Formation, and Lakota Formation have been interpreted to represent nonmarine deposition during Late Jurassic-middle Albian time. Strata deposited during late Albian-Campanian time, including the Fall River, New Castle, and Fox Hills Sandstones, record a change from fluvial deposition to marine conditions. The Lower Paleocene Fort Union Formation contains evidence for a return to fluvial or marginal marine conditions by early Cenozoic time.

Previous provenance work using stratigraphic thickness patterns infers that the sediments in the Upper Jurassic Morrison Formation were coming from the west. Based on sandstone petrography and heavy mineral analyses, sediment source regions for the Lower Cretaceous Lakota and Fall River Formations are interpreted to be the Transcontinental Arch to the east that spans from Minnesota to New Mexico, the upper Midwest region, and possibly the Canadian Shield. New detrital zircon data from Lower Cretaceous units across the Midcontinent support an easterly-derived sediment source. However, detrital zircon age spectra from other studies of the Lakota Formation and Morrison Formation in the Black Hills are described as being very similar to coeval strata located in Wyoming and Montana and are interpreted to represent a primarily western provenance. No known provenance interpretations have been published for the Upper Cretaceous or Lower Paleocene strata. Our new data will help to refine the sediment sources for strata deposited in the backbulge depozone of the Sevier foreland basin system.